This invention relates generally to magnetic resonance imaging (MRI) and spectroscopy, and more particularly the invention relates to self-refocusing RF excitation pulses with variable refocusing delay for use in MRI and spectroscopy.
Magnetic resonance imaging (MRI) is a non-destructive method for the analysis of materials and represents a new approach to medical imaging. It is completely non-invasive and does not involve ionizing radiation. In very general terms, nuclear magnetic moments are excited at specific spin precession frequencies which are proportional to the local magnetic field. The radio-frequency signals resulting from the precession of these spins are received using pickup coils. By manipulating the magnetic fields, an array of signals is provided representing different regions of the volume. These are combined to produce a volumetric image of the nuclear spin density of the body.
Briefly, a strong static magnetic field is employed to line up atoms whose nuclei have an odd number of protons and/or neutrons, that is, have spin angular momentum and a magnetic dipole moment. A second RF magnetic field, applied as a single pulse transverse to the first, is then used to pump energy into these nuclei flipping them over, for example to 90.degree. to 180.degree.. After excitation the nuclei gradually return to alignment with the static field and give up the energy in the form of weak but detectable free induction decay (FID). These FID signals are used by a computer to produce images.
Magnetic field inhomogeneity causes dephasing of nuclear spins after application of an RF excitation. Heretofore, application of a 180.degree. (.pi.) RF pulse has been used to refocus the spins and create a spin echo.
Self-refocusing excitation pulses can be designed to produce a spin echo at the end of the excitation pulse. See for example Geen and Freeman, "Band Selective Radiofrequency Pulses", Journal of Magnetic Resonance 93, pp. 93-141 (1991). Such band selective pulses can be a single suitably shaped amplitude modulated radiofrequency pulse.
A direct method of synthesizing self-refocusing RF excitation pulses has been developed. See Shinnar et al., "The Synthesis of Pulse Sequences Yielding Arbitrary Magnetization Vectors" Magnetic Resonance in Medicine, 12, pp. 74-80 (1989) and LeRoux "Exact Synthesis of Radiofrequency Waveforms", Proceedings of Seventh SMRM, pg. 1049, August 1988. The Shinnar-LeRoux (SLR) algorithm reduces RF pulse design to the design of two polynomials A.sub.n (z) and B.sub.n (z) where the magnetization M.sub.xy produced by an N sample RF pulse is: EQU M.sub.xy (X)=2A.sub.n.sup.* (Z)B.sub.n (Z)
where z=
e.sup.i.gamma.G.times.T/N and T is the pulse length.
Pauly, LeRoux, Nishimura and Macovski, "Parameter Relations for the Shinnar-LeRoux Selective Excitation Pulse Design Algorithm", IEEE Transactions on Medical Imaging, Vol. 10, No. 1, March 1991 presents a design of .pi./2 pulses by choosing B.sub.n (Z) to be a linear phase FIR filter, and A.sub.n (Z) to be the minimum phase polynomial consistent with B.sub.n (Z). This results in a minimum power RF pulse. The RF waveform is the inverse SLR transform EQU B.sub.1 (t)=SLR.sup.-1 {A.sub.n (Z),B.sub.n (Z)}.
A limitation in the known self-refocusing pulse lies in the spin echo occurring only at the end of the RF excitation pulse without a delay, thus eliminating the possibility of applying magnetic gradients prior to reception of the self-refocusing pulse.